Magnetic system for a stereo pick-up with a movable coil

ABSTRACT

In a stereo pick-up having movable coils (14) and an SmCo magnet (22) with a high energy content, with the coil system (14, 15, 18) arranged in front of one pole face of said magnet, a yoke plate (23) of a magnetic soft material is arranged in front of said system to increase the magnetic induction in the coils. The great coercive force of the magnet is utilized for simplifying and reducing the costs of the mechanical structure in that the yoke plate is formed with two legs (26) which extend rearwardly, engage and are secured to two opposite sides of the magnet. The short-circuit of the magnet established by these legs has no noticeable impact on the induction in the air gap because of the high coercive force of the magnet. The manufacturing costs of the magnet system are additionally reduced in that, instead of a hole to receive a mounting assembly (16) for the coil system, the magnet, whose material is very hard, is formed in its underside with a slot (30) in whose bottom the mounting assembly is arranged and secured by means of a plate-shaped projection (31) on a casing member (32) of plastics.

BACKGROUND OF THE INVENTION

Magnetic systems for pick-ups working according to the moving coilprinciple are usually constructed as a ring-shaped yoke circuitconsisting of a permanent magnet, pole shoes and an air gap in which thearmature or coil system of the pick-up is placed.

The air gap may be positioned at various locations in the magneticcircuit and thus utilize the magnet more or less efficiently,corresponding to greater or smaller flux density in the air gap with thesame magnetic volume.

Usually, the magnet is the most expensive component in the magnetcircuit, and until a few years ago mainly Alnico magnet types were used.These magnets are characterized by an energy product (BH)_(max) of 4 to10M gauss- sted for the anisotropic types corresponding to a magneticinduction B_(r) of about 7 to 13K gauss and coercive forces H from 0.7to 1.9K rsted. It is moreover characteristic of these magnets that witha square cross-section the length-width ratio will be>1 corresponding tobar shape owing to the relatively low coercive force. When these magnetsare loaded with a yoke circuit, the magnetic induction B_(r) declinesalong a working line depending upon load and magnet type. To reduce thecosts of magnet volume, the circuit is usually dimensioned to theworking point which gives (BH)_(max).

Owing to the elongate shape there will be a considerable loss, also inan incorporated state, because of stray fields around these magnets.Where possible, the magnet is therefore positioned preferably directlyup to the active air gap.

Recent years have seen the development of high energy magnets based onsamarium cobalt alloys with energy products of 20 to 27M gauss- rsted.

In particular the very high coercive force of 5 to 8K sted makes theminteresting for use in pick-ups. One reason is that the magnets giveapproximately the same B_(r) with a smaller volume, another is that thehigh coercive force entails that optimum dimensioning gives disc-shapedmagnets instead of bar-shaped ones, which is very expedient because ofthe miniaturization taking place within the pick-up field.

In moving coil pick-up systems, it is essential to have a powerfulhomogenous field in the air gap where the windings move.

The high energy magnets have the field radiation concentrated on thepole faces to a higher degree than an isotropic Alnico type, where amajor part of the radiation occurs from the sides at right angles tothese.

To utilize the properties of the high energy magnets optimally, at leastone of the pole faces of the magnet should restrict the air gap. The airgap induction is increased significantly by arranging a U-shaped yoke,illustrated in FIG. 2, in the usual manner, said yoke connecting the twopole faces with each other around the air gap. However, considerationsof space in moving coil phone cartridges and also process-technicalconsiderations in the production may cause another structure of themagnet circuit, which is the idea of the invention.

SUMMARY OF THE INVENTION

The invention concerns a magnet system of the type defined in theintroductory portion of claim 1, and its object is to provide such amagnet system which utilizes the mentioned properties in high energymagnets to simplify and thereby reduce the costs of pick-up magnetcircuits. This is important especially as these magnets are stillconsiderably more expensive than the Alnico types mainly because of theworking costs.

This object is achieved in that the pick-up is constructed as stated inthe characterizing portion of claim 1, since the high coercive force ofthe magnet makes it possible to place magnetically conductive parts atthe side or sides of the magnet, without the magnet being loadednoticeably. The yoke member does not have the ordinary yoke function ofdirecting the flux lines from one pole face back to the other pole face,but is merely to collect and concentrate the flux lines emanating fromthe pole face, so that the greatest possible part of these extendsthrough the intermediate armature. In the stated structure, the yokemember is fixed with respect to the magnet by simple means and in amanner which makes the assembling procedure more inexpensive, since themagnet, coil system and yoke can be mounted axially, which is anadvantage in automatic assembling procedures.

Thus, it is the idea of the invention that solely by mounting amagnetically conducting yoke plate at a short distance from one of thepole faces, such a concentration of the flux lines will be obtained fromthe pole face toward the yoke plate that the induction, depending uponthe distance between the mentioned faces, will be increased.

As stated in claim 2, also a permanent magnet may be used with itsopposite pole at the position of the yoke plate, resulting in an evenmore powerful air gap induction--also when no other return path is usedfor the flux than the air between the pole faces facing away from theair gap.

The great hardness of the magnet makes it difficult to drill such a holewith a small diameter with respect to the length for mounting ofattachment means for the armature and coil system as is present in thementioned known pick-up. A considerable manufacturing simplification canbe obtained when the magnet system has the structure stated in claim 3,as the track can relatively easily be produced by a rotating disc. Asuitable object, e.g. moulded in plastics, can then be placed in thistrack, in such a depth as to produce a through bore.

The invention will be explained more fully below with reference to thedrawing, in which

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1 and 2 show two generally known pick-up structures, partially insection,

FIGS. 3 and 4 are a bottom and a side view, respectively, of theessential parts of pick-up structures with their respective embodimentsof the magnet system of the invention, and

FIG. 5 is a perspective, partially exploded view of a complete pick-upwith the magnet system shown in FIG. 3.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

In FIG. 1, 10 is a bar-shaped permanent magnet on whose pole faces yokemembers 11 and 12 are placed. The ends of these define an air gap 13 inwhich an armature 14, provided with coils and secured to a stylus arm15, is movably positioned by means of a mounting assembly 16, introducedinto an axial hole 17 in the yoke member 12, and a damper pad 18. Thestylus arm 15 extends through a hole 19 in the yoke member 11.

The same reference numerals for corresponding parts are used in FIG. 2as in FIG. 1, viz. for the air gap 13, the armature 14, the stylus arm15, the mounting assembly 16, the hole 17, the damper pad 18 and thehole 19. The air gap 13 is here disposed between one end of a bar magnet20 and one end of a U-shaped yoke 21, whose other end engages the otherend of the magnet. Here, the hole 17 to receive the mounting assembly ofthe armature is thus formed in the magnet.

The armature, stylus arm and damper pad are also designated by 14, 15and 18, respectively, in the pick-up structure shown in FIGS. 3 and 4.In these structures, the armature assembly with damper pad is arrangedon one pole face of an SmCo magnet 22, which is made very short owing tothe high coercive force so that it is almost disc-shaped.

In FIG. 3, a yoke plate 23 is placed in front of the armature 14, saidyoke plate being a magnetic soft, high-saturable material, e.g. iron oran iron nickel alloy. This yoke plate collects and concentrates themagnetic flux lines and thus increases the induction between the airgap, here designated by 24, which is interposed between the yoke plateand the magnet pole face, so that the sensitivity of the pick-up isimproved correspondingly. The yoke plate 23 is secured in a simplemanner in that it is shaped with two legs 26 which extend rearwardly andengage the sides of the magnet 22 to which they are secured in asuitable manner.

An intermediate layer may optionally be provided between these legs andthe magnet. The circumstance that the legs 26 form magneticshort-circuit paths for the magnet has no greater impact on theinduction in the air gap 24 owing to the great coercive force of themagnet.

FIG. 4 shows a slightly amended embodiment where the yoke plate 23 hasjust one leg 27 which engages one side of the magnet 22.

No matter whether the yoke plate is shaped like an L or a U, the legs 26and 27 primarily serve to retain the yoke 23 with respect to the poleface 28 of the magnet. These side members are therefore not essentialparts of the magnet circuit, except that, of course, they give rise to acertain short-circuit of the poles 28 and 29 at the point where they aredisposed in close proximity to the pole faces. However, the highcoercive force of the magnet causes this load to have no importance. Theair gap with the high induction is exclusively formed by one pole face28 on the magnet and the yoke 23.

The armature system may be retained in the structures of FIGS. 3 and 4in a manner similar to that shown in FIGS. 1 and 2 by means of amounting system arranged in a hole in the magnet. Owing to the greathardness of the magnet, it is a difficult and rather expensive operationto drill a hole in the magnet with a small diameter with respect to thelength. FIG. 5 shows an embodiment of the structure of FIG. 3 which iseasier and cheaper to manufacture. Here, instead of a drilled hole, themagnet is formed with a slot 30 extending in the field direction in theunderside. Such a slot is relatively easy to produce with a rotatingdisc. The cylindrical mounting assembly 16 is arranged and retained inthe bottom of the slot 30 by means of a plate-shaped central projection31 on a casing member 32, which may be moulded in plastics andadditionally has two plate-shaped lateral projections 33 designed to bepushed in around and engage the outer sides of the legs 26 of the yokeplate 23. The upper edge face of the central projection 31 is formedwith a groove 34 of circular-arc shape in cross-section with the sameradius as the mounting assembly 16.

We claim:
 1. A magnet system for a stereo pick-up with a movable coiland containing a permanent magnet with a high energy product,characterized in that the magnet system comprises an L- or a U-shapedyoke member of magnetically conductive or permanent-magnetic material,and that one leg of said yoke member is disposed in opposite andsomewhat spaced as well as substantially parallel relationship with apole face of the magnet, while the other leg or legs are disposed inengagement with or closely adjacent to one magnet side or theirrespective magnet sides parallel with the field direction.
 2. A magnetsystem according to claim 1, characterized in that the yoke member isformed by a second permanent magnet with a high energy product and sooriented that the two pole faces directed toward each other haveopposite polarity.
 3. A magnet system according to claim 1,characterized in that the magnet is formed with a track extending in thefield direction from one pole face to the other.
 4. A magnet systemaccording to claim 2 characterized in that the magnet is formed with atrack extending in the field direction from one pole face to the other.